Slotted array antennas commonly use a waveguide distribution network for distributing RF energy to and from an array of slots placed along the broad wall of a waveguide channel. These waveguide-implemented antennas can be used for communication applications requiring space-limited mountings, such as in aircraft installations. In satellite communications applications, however, it is often a requirement that the antenna be capable of transmission and reception of signals having two different characteristic polarization states. This requirement can prove to be a significant obstacle to designing a space-limited slotted array antenna. Moreover, satellite applications often require a light-weight antenna design, capable of communicating signals with dual polarization states.
Dual polarization communication can be effected by the use of a pair of separate spaced-apart antennas, each having a corresponding polarization state different from the other. However, using a pair of differently polarized antennas often fails to satisfy the need to conserve installation space for a space-limited application. A space-saving alternative is to utilize a single slotted antenna to receive dual polarization signals, by implementing the concept of polarization diversity. Thus, a single slotted antenna capable of communicating signals with polarization diversity (having two characteristic polarization states) can obviate the need for two physically separated antennas.
A previously proffered solution for communicating information with dual characteristic polarization states is an interlaced combination of a pair of slot antennas. A first antenna having slots along the broad wall of a waveguide channel is integrated in a single antenna structure with a second antenna having slots along the narrow wall of a waveguide channel. The slots of the first antenna are associated with a particular polarization state, while the slots of the second antenna are associated with a separate polarization state. Although this interleaving of separate slot antennas into a single, integrated antenna structure can support the communication of dual polarized information, the antenna design also requires the use of end-feed networks with complex designs and interlaced antennas having different frequency responses. In addition, this stacking of broad and narrow wall waveguide channels in an interleaved manner can be difficult to manufacture. The interleaving of a pair of broad/narrow wall waveguide antennas to achieve the communication of dual polarized information generally results in increased design complexity and a difficult manufacturing process.
Another available dual polarized antenna comprises dual polarized slot radiators in bifurcated waveguide arrays. The radiating element comprises a pair of crossed slots in the side wall of a bifurcated rectangular waveguide that couples even and odd waveguide modes. One linear polarization is excited by the even mode, and an orthogonal linear polarization is excited by the odd mode. This antenna design suffers from the disadvantage of requiring an end-feed network rather than the preferred center or rear-feed network of typical slotted array antennas. In addition, manufacturing the antenna requires a relatively complex operation for cutting or stamping out the crossed-slot radiating elements in the side wall of the bifurcated rectangular waveguide.
Another prior antenna design relies upon a small circular hole or an "X"-shaped slot located in the broad wall of a rectangular waveguide, approximately half-way between the center line and the narrow wall. A righthand circular polarization can be achieved by feeding the waveguide from one end. In contrast, a left-hand circular polarization can be achieved by feeding the waveguide from the opposite end. This design suffers from the disadvantage of requiring two separate end-feed networks, rather than the preferred single center or rear-feed network of typical slotted array antennas.
Yet another antenna design communicates signals with dual simultaneous polarization states, by utilizing a cavity section positioned between input and output slots of a ridged-waveguide implemented slot radiator. The cavity section is effective to rotate the polarization of a signal with respect to the relative positions of the input and output slots. Thus, the shape of the cavity section can be utilized to rotate an electromagnetic field from a first polarization state to a second polarization state. In transmit mode, for example, the output slot will receive the electromagnetic signals having the second polarization state and radiate the electromagnetic signals into free space. Various shapes of the cavity section can be used to alter performance characteristics of the radiator, such as impedance matching. However, this design requires a feed network for feeding into the ridge side of the ridged waveguide. Such a feed network requires a complex design and an expensive machining operation. This design is also difficult to implement in a space and/or weight sensitive application, because the complex feed network adds thickness and weight to the overall antenna structure.
Therefore, there exists a need for a dual polarized slotted array antenna capable of supporting simultaneous dual polarization states while utilizing a conveniently manufactured and light-weight feed network. There also exists a need for a dual polarized ridged waveguide-implemented antenna employing a planar array of slots, which can be efficiently and readily manufactured using conventional manufacturing techniques. There is also a need for an improved waveguide slot radiator to support the reduction of the profile of a single structure slotted array antenna capable of supporting simultaneous dual polarization states.